1. Field of the Invention
An object of the present invention is a curved bar probe for an echograph. It finds application more particularly in the medical field where echographs are used for diagnostic purposes to reveal pictures of internal tissue structures of human bodies examined. Nevertheless it can be applied in all fields of utilization of ultra-sound, or where curved bars are used.
2. Discussion of Background
An echograph schematically comprises electrical generating means to produce an electrical signal that vibrates at an acoustic frequency. This signal is applied to an element or, rather, to a bar of piezoelectric transducer elements where it is converted into a mechanical excitation. The probe emits this mechanical excitation in a medium against which it is placed. Outside periods of transmission, the probe may be used to receive acoustic signals back-reflected by the medium and to convert these acoustic signals into electrical signals which can be introduced into reception elements. From the electrical signal thus received, it is possible to extract useful information, notably information capable of enabling the creation of an image. The quality of the image depends on the way in which the medium to be examined is explored.
Among the various possible solutions, exploration by sector scanning is presently one of the most efficient ones. To obtain it, it is enough to excite a group of adjacent elements in the bar, with predetermined delays with respect to one another, so as to focus the acoustic wave in one direction at transmission (the same organization of the delays is planned at reception to favour a signal coming from a given direction). By modifying the composition of the group of elements, by interrupting, for example, the supply to an element located on one side of the set, and by putting another element located on the other side into operation, and by re-organizing the delays for the new group thus formed, a new direction is obtained for the focusing of the acoustic signals. If all the piezoelectric elements are aligned with one another along a straight line, the scanning of the examined medium is a scanning by translation. On the contrary, if the bar of elements is curved, the scanning follows the perpendicular to the tangent to the curve formed by the arrangement of the elements: it is sectorial if this curve is an arc of a circle.
The making of curved bar probes is conventionally done in the following way. A support is used, with a relatively small thickness, for example 2 to 3 mm, and made of a flexible material. Then, a small bar of piezoelectric crystal is fixed to this support. By cutting out operations, in particular with a saw, a partition is made in this small bar, so as to divide it into several piezoelectric elements. The partition is done in such a way that, between each element, the support is not cut. Each element remains fixed to the support. Since the support is flexible, it is then enough to fix it to a base with an appropriate curved shape to obtain a desired curved bar. In a European patent application No. 84 308 373.4, filed on Dec. 3, 1984, an embodiment of this type is described.
This approach, however, has a drawback. In effect, owing to the elastic nature of the support, it cannot be kept curved except by exerting a permanent holding force. As indicated by the document referred to, this force can be obtained by bonding the support to the base. The face of the piezoelectrical elements in front of the place where the useful acoustic waves are propagated, is called the front face, the face opposite the front face is called the rear face. During transmission, the transmitted acoustic wave is propagated, in principle, in both directions: frontwards and rearwards. Only the wave transmitted frontwards is useful. Steps are taken, accordingly, to prevent the disturbances due to the rear wave. In particular, action is taken on the acoustic impedance of the support and the base to prevent the rear wave from being reflected towards the bar. Now, this can be achieved only imperfectly once the support has a bonding surface on the base. This bonding surface reacts all the more strongly as the support is thin so as to be capable of being deformed and, hence, as this surface is close to the piezoelectric small bar proper.
Despite changes in the composition of the bonder used to join the support to its base, as indicated in the document referred to, the most efficient result is not obtained. Furthermore, the bonding operations are never perfect, and the solidness of the unit may be thereby affected. Finally, the very nature of the flexible elements used to this end is not favourable to the choice, for the material of the support, of good acoustic impedance. For, there are known ways of bringing the acoustical impedance of materials into by making mixtures or by including therein micro-beads made of plastic or glass for example. And flexible synthetic materials are ill-suited to this operation.
The characteristics of the introduction of claim 1 are known from the Japanese abstract 57181299. From the document EP-A-l28 049, the making of a probe comprising the material, polymerizable epoxy resin, is known. The backing known from this document is bonded to the probe by a layer.
The probe according to the invention is characterized by the characteristics of the characterizing part of claim 1. The use of a probe of this type, comprising a cold polymerizable resin, enables the resolving of the problem of subsequently positioning the base (by the intimate adhesion of the two resins) without this positioning forming an echo surface for the rear acoustic wave.